The term "superconductivity" is applied to the phenomenon of immeasurably low electrical resistance exhibited by materials. Until recently superconductivity had been reproducibly demonstrated only at temperatures near absolute zero. As a material capable of fexhibiting superconductivity is cooled, a temperature is reached wat which resistivity decreases (conductivity increases) markedly as a function of further decrease in temperature. This is referred to as the superconducting transition temperature or, in the context of superconductivity investigations, simply as the critical temperature (T.sub.c). T.sub.c provides a conveniently identified and generally accepted reference point for marking the onset of superconductivity and providing temperature randkings of superconductivity in differing materials.
It has been recently recognized that certain rare earth alkaline earth copper oxides exhibit superconducting transition temperatures well in excess of the highest previously known metal oxide T.sub.c, a 13.7.degree. K. for the metal Nb.sub.3 Ge.
Recent discoveries of higher superconducting transition temperatures in rare earthf alkaline earth copper oxides are reported in the following publications:
P-1 J. G. Bednorz and K. A. Muller, "Possible High T.sub.c Superconductivity in the Ba-La-Cu-O System", Z. Phys. B.--Condensed Matter, Vol. 64, pp. 189-193 (1986) revealed that polycrystalline compositions of the formula Ba.sub.x La.sub.5-x Cu.sub.5 O.sub.5(3-y), where x=1 and 0.75 and Y&gt;O exhibited superconducting transition temperatures in the 30.degree. K. range.
-2 C. W. Chum, P. H. Hor, R. L. Meng, L. Gao, Z. J. Huang, and Y. Q. Wang, "Evidence for Superconductivity above 40.degree. K. in the La-Ba-Cu-O Compound System", Physical Review Letters, Vol. 53, No. 4, pp. 405-407, Jan. 1987, reported increasing T.sub.c to 40.2.degree. K. at a pressure of 13 kbar. At the end of this article it is stated that M. K. Wu increased T.sub.c to 42.degree. K. at ambient pressure by replacing Ba with Sr.
P-3 C. W. Chu, P. H. Hor, R. L. Meng, L. Gao, and Z. J. Haung, "Superconductivity at 52.5.degree. K in the Lanthanum-Barium-Copper-Oxide System", Science Reports, Vol. 235, pp. 567-569, Jan. 1987, a T.sub.c of 52.5.degree. K. for (La.sub.0.9 Ba.sub.0.1).sub.2 CuO.sub.4-7 at high pressures.
P-4 R. J. Cava, R. B. vanDover, B. Batlong, and E. A. Rietman, "Bulk Superconductivity at 36.degree. K. in La.sub.1.8 Sr.sub.0.2 CuO.sub.4 ", Physical Review Letters, Vol. 58, No. 4, pp. 408-410, Jan. 1987, reported resistivity and magnetic susceptibility measurements in La.sub.2-x Sr.sub.x CuO.sub.4, with a T.sub.c at 36.2.degree. K. when x=0.2.
P-5J. M. Tarascon, L. H. Green,e W. R. Mckinnon, G. W. Hull, and T. H. Geballe, "Superconductivity at 40.degree. k. in the Oxygen-Defect Perovskites La.sub.2-x Sr.sub.x CuO.sub.4-y ", Science Reports, Vol. 235, pp. 1373-1376, Mar. 13, 1987, reported title compounds (0.05.ltoreq..times..ltoreq.1.1) with a mxaimum T.sub.c of 39.3.degree. K.
P-6 M. K. Wu, J. R. Ashburn, C. J. Torng, P. H. Hor, R. L. Meng, L. Gao, Z. J. Hunang, Y. Q. Wang, and C. W. Chu, "Superconductivity at 93.degree. K. in a New Mixed-Phase Y-Ba-Cu-O Compound System at Ambient Pressure", Physical Review Letters, Vol. 58, No. 9, 908-910, Mar. 2, 1987, reported stable and reproducible superconducting transition temperatures between 80.degree. and 93.degree. K. at ambient pressures for materials generically represented by the formula (l.sub.1-x M.sub.x).sub.a A.sub.b D.sub.Y, where L=Y, M=Ba, A=Cu, D=O, x=0.4, a=2, b=1, and y.ltoreq.4.
The experimental details provided in publications P-1 through P-6 indicate that the rare earth alkaline earth copper oxides prepared and investigated were in the form of cylindrical pellets produced by forming an intermediate oxide by firing, grinding or otherwise pulverizing the intermediate oxide, compressing the particulate intermediate oxide formed into cylindrical pellets, and then sintering to produce a polycrystalline pellet. While cylindrical pellets are convenient articles for cooling and applying resistance measuring elecrodes, both the pellets and their preparation prcedure offer significant disadvantages to producing useful electrically conductive articles, particularly articles whichy exhibit high conductivity below ambient temperature--e.g., superconducting articles. First, the step of grinding or pulverizing the intermediate oxide on a commercial scale prior to sintering is both time and energy consuming and inherently susceptible to material degradation due to physical stress on the material itself, erosion of grinding machinery metal, and handling. Second, electrically conductive articles rarely take the form of pellets. Electrically conductive articles commonly include either thin or thick films forming conductive pathways on substrates, such as insulative and semiconductive substrates--e.g., printed and integrated circuits.